1. Field of the Invention
The invention relates to an apparatus for forming magnetic resonance images, which apparatus includes
a gradient coil system that includes a carrier on which gradient coils are arranged, said gradient coils being attached to a frame of the apparatus by way of connection means,
which connection means are constructed so as to reduce the transfer of mechanical vibrations that are produced by the gradient coil system.
2. Description of the Prior Art
An MRI apparatus of this kind is known from U.S. Pat. No. 5,793,210. The MRI apparatus that is described in the cited document is provided with a gradient coil system with gradient coils that are arranged in an enclosure in which the gas pressure is lower than that of the ambient atmosphere. Coils of this kind may be arranged on a carrier which itself is attached to further parts of the MRI apparatus; the carrier is attached notably to the frame of the MRI apparatus by way of connection means.
It is a generally known fact that gradient coils in operation produce noise that is very annoying to the patients to be examined. Therefore, the technical aim is to reduce this noise as much as possible. To this end, the gradient coils in the known MRI apparatus are arranged in a vacuum atmosphere with a residual pressure such that the acoustic transfer of vibrations, arising in the gradient coils, to the surroundings is strongly reduced. Said vacuum space may be filled with a noise absorbing fiber glass material so as to reduce the transfer via this atmospheric path even further. In order to counteract the transfer of vibrations via the connection means, the connection means are constructed so as to mitigate the transfer of the vibrations that are generated by the gradient coil system. The gradient coil system is supported notably by materials that have the desired acoustic properties, for example rubber, plastic or an epoxy material, or by resilient elements that, like said materials, bear on rigid structural parts such as supports or flanges that are especially provided for this purpose.
Said known steps all have restrictive drawbacks. Said materials dampen the acoustic vibrations to a limited extent only and still allow an acoustic path through the surrounding atmosphere. Enclosing the gradient coils by means of a noise absorbing fiber material also dampens the acoustic vibrations to a limited extent only and still leaves an acoustic path through the connection means; this also holds when the gradient coils are arranged in vacuum. Moreover, arranging the gradient coil in a vacuum envelope necessitates drastic structural steps to be taken and also requires additional space; the latter is undesirable notably in the vicinity in which the coils of the MRI apparatus are situated. A flexible suspension of the gradient carrier also has the drawback that in a macroscopic sense a change of position of the gradient coils could then occur; this drawback is not imaginary, because the design of a flexible suspension always aims for maximum vibration isolation, so an as flexible as possible suspension. A macroscopic change of position has an adverse effect on the quality of imaging.
It is an object of the invention to provide an MRI apparatus of the kind set forth in which acoustic propagation to the environment of vibrations that are produced in the gradient coil system is counteracted in a different manner.
To this end, the MRI apparatus in accordance with the invention is characterized in that the connection means include a number of suspension elements, a first end of which is attached to the carrier whereas another end is attached, directly or indirectly, to the frame of the MRI apparatus, said connection means having a first mechanical stifffiess in a first direction and a second, smaller stiffness in at least one direction that extends perpendicularly thereto, each of the first ends of the suspension elements being attached to a point of attachment of the carrier such that this point exhibits at least one low-vibration direction, and the connection direction between the first end and the second end of the suspension element being substantially coincident with the low-vibration direction of the point of attachment of the carrier.
The stiffness in the first direction is determined by the requirement imposed as regards positional stability, meaning that the gradient carrier must retain its position during operation from a macroscopic point of view. The stiffness in the second direction must then be much smaller than that in the first direction. The simplest form of such a suspension element has the shape of a rod that has a thickness such that its bending stiffness is much smaller than the stiffness in the axial direction, or of a rod that is attached to the surroundings in such a manner that it is capable of tilting about its point of attachment. In the latter case the transverse stiffness is even negligibly small relative to the axial stiffness. Another form of such a suspension element can be obtained by way of a notch hinge that is proportioned in such a manner that a small hinge stiffness about one axis is obtained, or by way of a combination of two notch hinges that are proportioned such that a small hinge stiffniess about two mutually perpendicular axes is obtained. Other structural elements that are known per se are also feasible for as long as the requirement is satisfied that such an element should exhibit a given degree of mechanical stiffness in one direction and a much smaller stiffness in a direction perpendicular thereto.
Generally speaking, the gradient coil system includes a carrier on which the gradient coils are mounted. The carrier is often shaped as a cylinder in which the gradient conductors and the carrier are united so as to form one rigid unit. Using said suspension elements, the location in the space of the carrier, that is, of the entire gradient coil system can be defined. Generally speaking, each point of a member that produces vibrations exhibits vibration deflections in three non-coincident directions which, moreover, exhibit mutual phase differences. The invention is based on the recognition of the fact that there are locations on a gradient carrier, notably a gradient carrier having a cylindrical shape, where in one vibration direction the amplitude is significantly lower than in the other directions. The former direction is referred to as the low-vibration direction. When the suspension element in aaccordance with the invention is attached in such a location (that is, in a location where the amplitude in one vibration direction is substantially lower than that in the other directions) and when the low-vibration direction is chosen as the direction of said suspension element, the occurrence of vibrations in the longitudinal direction of the suspension element will be much less than in the other directions, and the vibrations in said other directions will be transferred to a very small extent only by the suspension elements because these elements have a small stiffness in said directions. The transfer of the vibrations produced by the gradient coil system is thus strongly reduced. The use of a suspension element in accordance with the invention also offer the advantage that this suspension is highly unsusceptible to deviations of the axial suspension direction from the desired direction, that is, the low-vibration direction of the point of attachment. It can be demonstrated that the deviation of the reduction factor of the vibration forces that is transferred by the suspension element in the case of misalignment in the axial direction is proportional to the square of the sine of the angle of deviation. For small angles this results in a very small deviation of the reduction factor.
Preferably, for at least one of said suspension elements the ratio of the axial stiffness to the transverse stiffness is greater than 50:1. It has been found that this numerical value yields an acceptable vibration isolation for the remaining vibrations.
It may occur that the effect of the described steps in accordance with the invention is less for vibration frequencies other than the most important frequency, that is, for the frequency at which the largest acoustic energy occurs. This means that no low-vibration direction is present in the point of attachment for said other frequencies, or that a low-vibration direction that is present does not coincide with the longitudinal direction of the suspension element in accordance with the invention. In order to realize a further vibration isolation also for one or some of said other frequencies, at least one of said suspension elements is provided with an active drivable element for virtually reducing the stiffness of said suspension element in the axial direction. Said element can now be controlled in such a manner that the longitudinal displacement in the suspension element that is caused by the residual vibration is compensated by an opposed extension/reduction of the length of the active drivable element that is caused by a variation of the length of the controllable element that is induced by the driving. This results in a virtual reduction of the stiffness of the suspension element for the drive frequency (frequencies). Such an element may be formed, for example by a piezoelectric actuator or by an electromagnetic actuator. This step counteracts the transfer of said residual vibrations (in the form of forces that act on the other structural parts of the MRI apparatus).
In conformity with a preferred embodiment of the invention, the MRI apparatus is provided with a drive circuit for driving the active drivable element, which drive circuit includes a feedback circuit that is arranged between a force sensor that is provided at the area of the relevant point of attachment of the carrier and the active drivable element. The force sensor measures the transferred force in the point of attachment, notably the force in the longitudinal direction of the suspension element. The force transferred in the longitudinal direction can thus be controlled so as to be zero.
In conformity with a further embodiment of the invention, the MRI apparatus is provided with a gradient control circuit that is intended to produce the signal that generates the gradient field of the MRI apparatus, and also with a drive circuit for driving the active drivable element, which drive circuit includes a feed-forward circuit that is connected between the gradient control circuit and the active drivable element. This embodiment advantageously utilizes the a priori knowledge concerning the state of vibration of the gradient coil system. This knowledge is derived from the control signal for the gradient currents, that is, in such a manner that there is generated a compensation signal for the active drivable element, the compensating effect of the drivable element being the same as when use is made of a feedback circuit.
Another embodiment yet of the MRI apparatus in accordance with the invention is provided with a cylindrical carrier for the gradient coils, which carrier is attached to four of said suspension elements at a first end and to two further suspension elements of this kind at a second end. A gradient coil system can thus be realized with suspension elements that act as ideal notch hinges; this means that the stiffness in the directions perpendicular to the longitudinal axes is of a value that is negligibly small for all practical purposes. Consequently, in a situation where even a slight transfer of vibrations in the transverse direction is undesirable, such a transfer is optimally counteracted. Moreover, macroscopic low-frequency motions (for example, of the order of magnitude of 10 Hz) of the carrier are also effectively counteracted by such a suspension by means of six suspension elements, so that deterioration of the imaging quality of the MRI apparatus is avoided.
In another embodiment yet of the MRI apparatus in accordance with the invention at least one of said suspension elements is constructed in the form of a rod-shaped element that comprises a thickened central portion. The effect of this step in accordance with the invention consists in that the thickened portion enhances the collapsing stability of the rod. As a result of the presence of this thickened portion, the remaining, non-thickened portions may be constructed so as to be thinner than when the thickening of the central portion were omitted, so that the transverse stiffness is even further reduced.